It is already known that fuels for internal combustion engines, in particular diesel fuels, may have very different densities or heating values. Diesel fuels are usually mixed with additives which may in particular influence the ignition performance, anti-friction and lubricating qualities and the exhaust emissions. A measure of the quality of the ignition performance of diesel fuels is for example the cetane number CZ which is determined on a specially constructed single-cylinder test engine. Comparative fuels, which are compared with the very ignitable n-cetane C16H34 and the less ignitable α-methyl naphthalene, can be tested on this test engine. The cetane number is fixed in a range between 0 and 100. A cetane number that is too low pollutes the environment, particularly by forming carbon particulate matter in the case of diesel engines. The cetane number CZ is conventionally between 50 and 55 in Germany.
The cetane number of diesel fuels can differ greatly, it is relatively low in the case of bio-diesel in particular. A further problem lies in the fact that when refuelling, the fuels in the fuel tank are mixed with different qualities. As a result it is very difficult for the motor control unit, for example in the case of a common rail injection system, to control a quantity of fuel to be injected in such a way that optimum combustion results with minimal exhaust gas emissions in particular. Added to this is the fact that in the case of a fuel with a lower density, the energy supplied to the internal combustion engine is less than in the case of a fuel with a higher density. The engine performance is also affected to an undesirable extent as a result. A motor control unit must take all of these factors and the current operating parameters into consideration such that an optimum quantity of fuel to be injected can be calculated in the case of a corresponding torque requirement. With known motor control units these requirements are only insufficiently satisfied, however.
Previously this problem has been solved, in particular in the case of diesel fuels, by fitting an optical sensor in the fuel tank. The optical sensor has the task of inferring the quality from the fuel's opaqueness. However this method is relatively unreliable as individual fuel producers add a dye to the fuel. As a result an objective assessment of the fuel used is rendered considerably more difficult.